Web-based health behavior change interventions - including smoking cessation programs - offer great promise but in many cases their absolute efficacy is diminished because many participants often spend relatively little time accessing program content. One potential solution involves adding treatment components that are delivered to participants' mobile phones. More than 90% of Americans use mobile phones and most keep their phones in close proximity throughout the day. As a result, these mobile devices can provide just in time assistance when it is most needed and they are proactive in that they reach out to users to deliver content and to increase its utilization and salience. Objectives: We propose a controlled test assessing tobacco abstinence associated with an adjunctive Mobile intervention combined with a best-practices Web-based smoking cessation intervention. We hypothesize that this Web Mobile approach will yield greater efficacy than a Web Only condition. Methods: We propose a 2-arm RCT with 2,260 adult study participants randomized to a Web Mobile or a Web Only condition. Because the central question is whether the combined Web Mobile approach improves efficacy, both conditions use a very similar best-practices core website that embodies many of the features found in the enhanced interventions tested in our prior Web-based tobacco cessation trials. The mobile modalities include automated text messages (SMS) and automated phone calls (IVR or Interactive Voice Response). Measures: Our primary outcome is sustained tobacco abstinence operationally defined as no tobacco use during the interval defined from the 3-month assessment to the 6-month assessment. At both the 3- and 6- month follow-up assessments we measure 7- and 30-day point prevalence tobacco abstinence. We also compare the intervention cost and incremental cost-effectiveness (cost per-participant and cost per-quit) of each experimental condition. Ancillary analyses will examine potential predictors, moderators, and putative mediators (i.e., self-efficacy, program use, use of pharmacological adjuncts). Benefits: To the best of our knowledge, this application describes the first RCT that assesses the incremental efficacy of adding Mobile components to a Web-based smoking cessation intervention. Evaluating the value of adding Mobile components to a Web intervention will have substantial public health impact because such interventions are able to cost-effectively reach many smokers who want to quit. The research will also advance the emerging science of Web Mobile interventions by seeking to identify beneficial adjunctive mechanisms.